This invention concerns an infrared detector using a pyroelectric device composed of a high polymer material and, more specifically, it relates to an infrared detector capable of effectively processing electric signals obtained from the pyroelectric device.
As known to the relevant field of art, the state of polarization in a pyroelectric device is changed with the changes in the temperature therein, and heat radiation such as of infrared rays to the pyroelectric device can be detected by taking out the changes in the polarization as electric signals. Since the electric signals taken out correspond to the changes in the temperature of the device, however, after the temperature in the device has been settled to a constant temperature in stationary infrared irradiation to the device, electric signals capable of detecting the presence or absence of the infrared irradiation can no more be taken out from the device. In view of the above, conventional infrared detectors using a pyroelectric device are generally provided with a chopper on the infrared irradiation side so as to apply intermittent infrared irradiation to the irradiated surface of the pyroelectric device for obtaining electrical signals capable of detecting infrared irradiation even in the stationary infrared irradiation.
In addition, since the pyroelectric device generally has an electric capacitivity and a high internal impendance, the device is generally used in combination with an impedance conversion circuit comprising a field effect transistor (FET) for the procession of electrical signals taken out from the device.
While pyroelectric devices composed of high polymer substance and having very excellent pyroelectric property have been known and application of them to the infrared detector has been proposed recently, mere combination of such a pyroelectric device and an impedance conversion circuit comprising a field effect transistor can not provide effective utilization of the excellent pyroelectric property of the device, and the electric signals obtained from the pyroelectric device can not be issued from the impedance circuit depending on the case. Upon intermittent infrared irradiation by way of the chopper to the pyroelectric device, the pyroelectric device produces an electric signal acting as a bias voltage as well as electric signals corresponding to the intermittent infrared irradiation. If the electric signal acting as the bias voltage goes near or lower than the pinch-off voltage of the N-type field effect transistor, the output electric signals corresponding to the intermittent infrared irradiation are clipped or no output electric signals are issued at all. The above phenomenon is particularly remarkable in the pyroelectric devices of excellent pyroelectric property and, therefore, their excellent property can not effectively be utilized.
An object of this invention is to provide an infrared detector capable of converting the internal impedance of a pyroelectric device and effectively processing the electrical signals taken out from a pyroelectric device.
Another object of this invention is to provide an infrared detector capable of effectively processing electrical signals taken out from a pyroelectric device having an excellent pyroelectric property.
Another object of this invention is to provide an infrared detector capable of converting the impedance in a simple structure.
Another object of this invention is to provide an infrared detector capable of generating electric signals ranging from dc current to high frequency ac current as it is that can be obtained from the pyroelectric device.
Another object of this invention is to provide an infrared detector keeping electric signals issued during operation from the impedance conversion circuit from being clipped.
An additional object of this invention is to provide an infrared detector in which a pyroelectric device and an impedance conversion circuit can be integrated in a small size.